The storage of electrical energy has become increasingly important in recent decades. Electrical energy may be stored with the aid of batteries. Batteries convert chemical reaction energy into electrical energy. A distinction is made between primary batteries and secondary batteries. Primary batteries are non-rechargeable, while secondary batteries, also referred to as accumulators, are rechargeable. A battery includes one or multiple battery cells.
In particular so-called lithium-ion battery cells are used in an accumulator. They are characterized, among other features, by high energy densities and an extremely low self-discharge.
Lithium-ion battery cells include a positive electrode (cathode) and a negative electrode (anode). The positive and negative electrodes each include a current collector, to which a positive or negative active material (cathode or anode active material) is applied. The positive and negative active material is characterized in particular in that it is capable of reversible intercalation and deintercalation of lithium ions.
The active material for the negative electrode (anode active material) is, for example, silicon which may form compounds with lithium atoms. However, carbon compounds such as graphite are also widely used as active material for negative electrodes. Lithium atoms are intercalated into the active material of the negative electrode in the charged state.
Lithium-containing transition metal oxides or lithium-containing transition metal phosphates which are able to reversibly intercalate and release lithium ions are generally used as active material for the positive electrode (cathode active material). In particular in applications in which a high energy density is necessary, so-called high-energy materials such as high-energy (HE) nickel-cobalt-manganese (NCM) electrodes (for example, x LiMO2·(1−x) Li2MnO3, where M=Ni, Co, Mn) are used. A generic battery that uses such an HE-NCM electrode is described in German Patent Application No. DE 10 2012 208 321 A1, for example.
During operation of the battery cell, i.e., during a discharging operation, electrons flow in an external circuit from the negative electrode to the positive electrode. During a discharging operation, lithium ions migrate from the negative electrode to the positive electrode within the battery cell. In the process, the lithium ions are reversibly deintercalated from the active material of the negative electrode, also referred to as delithiation. During a charging operation of the battery cell, the lithium ions migrate from the positive electrode to the negative electrode. In the process, the lithium ions are reversibly reintercalated into the active material of the negative electrode, also referred to as lithiation.
The electrodes of the battery cell have a foil-like design and are wound to form an electrode winding, with a separator situated in between which separates the negative electrode from the positive electrode. Such an electrode winding is also referred to as a “jelly roll.” The electrodes may also be layered one above the other to form an electrode stack.
The two electrodes of the electrode winding or of the electrode stack are electrically connected with the aid of collectors to poles of the battery cell, also referred to as terminals. A battery cell includes an electrode winding or electrode stack.
The electrodes and the separator are surrounded by an electrolyte which is generally liquid. The electrolyte is conductive for the lithium ions, and allows transport of the lithium ions between the electrodes.
U.S. Patent Appl. No. 2015/0064537 A1 describes a lithium-ion battery that includes a positive electrode, a negative electrode, and a solid electrolyte situated between these electrodes, the solid electrolyte being provided with a protective layer on at least the surface facing the positive electrode. The protective layer is used to prevent electrical contact of the electrodes with the electrolyte. A material that includes Al2O3 and Si3N4 is provided for the protective layer.
German Patent Application No. DE 10 2013 204 671 A1 describes an electrode for an electrochemical energy store, including a layer of an active material, and a protective layer which is at least partially applied to the active material and which includes a material based on fluorophosphate.
Conventional electrodes use active materials that frequently enter into reactions upon contact with the electrolyte under the operating conditions of the lithium-ion cells. This may result in changes in the active material and in the electrolyte. In particular at high voltages, for example at voltages ≥4.3 V, this may result in changes in the active material of the electrode (in particular the positive electrode) and in the electrolyte (oxidation of the electrolyte at the cathode active material). The operation of the cell thus results in a loss of the capacity of the cell (so-called “capacity fade”) and an increase in the internal resistance of the cell. The object of the present invention, therefore, is to provide an electrode having a preferably constant capacity and internal resistance, even after a long service time of the cell.